1. Field of the Invention
The present invention relates to a circuit board including a multilayer resin film for use in electronic equipment or the like.
2. Description of the Related Art
To lighten, thin, shorten, and miniaturize electronic equipment is particularly desired in household electrical appliances. Such electrical appliances are achieved by reducing the size of their components. An electronic circuit board of the components, is generally formed of glass epoxy board. When high density is required for a circuit board of miniaturized electronic equipment, the circuit board is formed in a multilayer fashion or together with a flexible board, a hybrid IC or the like.
However, the manufacturing of circuit boards, where low cost is desired, a complicated process is used to form through holes in a multilayer (not less than four layers) glass epoxy board, which results in remarkably high cost. An expensive material may be used as the flexible board because of its heat resistance. The above stated problems illustrate the need to improve high density circuit boards.
A circuit board formed of a multilayer resin film has been developed to resolve these problems. It has features of a simple manufacturing process, low cost. three-dimensional wiring.
FIG. 3 (Prior Art) illustrates a conventional circuit board in which resin films 1 have not yet been laminated one on top of another. The resin films 1 have heat deformation properties. Through holes 2 are formed at predetermined positions of the resin films. Conductive patterns 3 and electrodes 4 for connecting the resin films to mounting parts are formed on the surfaces and undersurfaces of the resin films by printing. The resin films 1, conductive patterns 3 and electrodes 4 are heated and pressed by fixed plates 5 of a thermal press (not shown) at a fixed pressure to integrally form a circuit board 6 (see FIG. 4 (Prior Art).
Unlike a normal glass epoxy board, a conductive portion of the circuit board 6 is formed by integrally forming the conductive patterns 3, electrodes 4 and fixed plates 5 by the thermal press. The conductive patterns of each of the resin films are connected to each other by integrally forming the resin films by the thermal press. In other words, they are connected by the flow of resin of the films. The through holes serve to connect the conductive patterns to each other or connect the conductive patterns to the electrodes in the circuit board and are simultaneously formed in a single process. The manufacture of the above-described circuit board does not require such a complicated plating process, as is required for that of the glass epoxy board. The circuit board can thus be manufactured at low cost.
In the manufacture process of the circuit board described above, when the resin films and conductive patterns are thermally pressed, the lower resin films and their conductive patterns are caused to flow by plastic deformation of resin and swell over the through holes (swelling effect).
The conductive patterns protruding from below are connected to the lands (electrodes) around the upper through holes. It is therefore desirable that the resin of the resin films and conductive patterns is sufficiently deformed to connect the conductive patterns to each other.
As described above, since the resin films are connected by deforming the conductive patterns of through hole portions when the films are thermally pressed, the upper and lower conductive patterns, including the resin films, need to be sufficiently deformed. However, the conductive patterns other than those of the through hole portions are also deformed when the resin films are thermally pressed. There is a possibility that the conductive patterns are deformed in their lateral direction as well as in their thickness direction. The intervals between the conductive patterns are not regularly maintained; accordingly, the reliability of the circuit board is lowered.